Demonstration Corner

Anisotropy is a factor that needs to be taken into consideration when measuring the thermal conductivity of a material. Anisotropy occurs when the thermal conductivity of a material is directionally dependent, and conducts heat at different rates in different directions. A material that is isotropic will have the same thermal conductivity results regardless of the direction of travel. Many common materials around us are anisotropic, one excellent example is wood. Just by looking at wood it is obvious that it has fibers running in a specific direction, referred to as the grain. Studies performed on the thermal conductivity of wood show that heat conducts much more quickly along the grain than across it. Anisotropic properties have implications in how a material is used, as for many materials how they conduct heat is crucial to their performance. Developers need to know exactly how a plastic component is going to move heat away from an electronic, or how a material used in a high temperature application is able to conduct and withstand heat. Torlon is a polyamide-imide, a type of polymer, that has an incredible range of applications due to its high strength and stiffness, in addition to its ability to retain its characteristics at high temperatures up to 275°C. Here at Thermtest, we set out to measure the thermal conductivity of a sample of Torlon using the Hot Disk TPS 2500 S, with the goal of not only obtaining those values, but also to determine whether it was an isotropic or anisotropic material.

Figure 1. Torlon can come in many different forms, in this case it is in plates1.

Thermtest’s scientist used the single sided sensor, the TPS-S, to test the thermal conductivity of Torlon. The TPS-S is capable of the same range of measurements as the Hot Disk two sided sensor, and is an excellent option for clients who have expensive, hard to cut, or rare samples to be tested. A single Torlon piece was placed on top of the TPS-S, and a weight was placed on the free side of the Torlon in order to promote good thermal contact with the sensor (Figure 2). The sensor was attached to the Hot Disk TPS 2500 S, a powerful thermal conductivity instrument capable of testing thermal conductivity between 0.005 and 1000 W/mK. The TPS 2500 S performs isotropic and anisotropic testing, measures thermal conductivity, thermal diffusivity, and specific heat simultaneously and works with solids, liquids, pastes and powders. All tests were 20 seconds in length and used a power of 0.1 W.

Figure 2. Photo of the set up used to test the thermal conductivity of Torlon using the TPS-S in the Thermtest lab.

Initial tests were performed for bulk (isotropic) measurement, while further testing used the anisotropic software module and tested for directional thermal conductivity and thermal diffusivity. The anisotropic module is easy to use, and runs on the assumption that the main directions in the material are orthogonal, and can be described in terms of a, b, and c axes. Furthermore, the properties along the b and c axes, that is in the planar direction, must be the same, while those along the a axis, or the axial direction, are different (Figure 3). The sample is placed parallel to the planar axis and perpendicular to the axial axis, therefore the sample set up is the same as it was for isotropic testing (Figure 2).

Figure 3. Diagrams illustrating how heat moves in the planar direction (in plane) on the left, and in the axial direction (through plane) on the right.

The initial results obtained by testing for bulk thermal conductivity are presented in Table 1. Upon comparison with a literature reference value of 0.540 W/mK produced by following ASTM C177, our scientist concluded that their result of 0.901 W/mK was much higher than it should be. Hence, the decision to test for anisotropy was made. The anisotropic module available with Hot Disk software tests for through plane and in plane thermal conductivity. The results of these tests are presented in Table 2. From comparing our experimental results to the standard reference value, it is clear that the reference matches the thermal conductivity value obtained for through plane directional testing (0.550 W/mK). Based on these results, our scientist concluded that Torlon is indeed an anisotropic material.

Table 1. Results produced by the Hot Disk TPS 2500 S from bulk thermal conductivity and diffusivity testing of Torlon.

This experimental project is an excellent example of the range of applications that the Hot Disk TPS thermal conductivity equipment can be used for. In this case, the equipment not only produced the thermal conductivity and diffusivity of a polymer, but also enabled our scientist to determine if the sample was isotropic or anisotropic. This ability to characterize materials gives the TPS an additional range of use that is beneficial to industry and academia alike.

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